The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) for improving the UMTS standard to cope with future requirements in terms of improved communication services such as higher data rates, improved efficiency, and lower costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS, and Evolved UTRAN (E-UTRAN) is the radio access network of a LTE system. In a UTRAN and E-UTRAN, a user equipment (UE) device is wirelessly connected to a Base Station (BS), commonly referred to as a NodeB or an evolved NodeB (eNodeB). Each BS serves one or more areas referred to as cells.
The possibility of identifying the geographical location of users in the wireless networks has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, and emergency services. Different services may have different positioning accuracy requirements imposed by the application. Furthermore, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, such as E911 from the Federal Communications Commission (FCC) in the United States and the corresponding E112 standard in Europe.
In many environments, a user's position may be accurately estimated by using positioning methods based on Global Positioning System (GPS). However, GPS is known to be associated with high costs due to higher UE complexity, a relatively long time to a first positioning fix, and a high UE device energy consumption due to a need for large computational resources, resulting in rapid battery drain. Today's networks often have the capability to assist UE devices to improve the terminal receiver's sensitivity and the GPS startup performance through Assisted-GPS (A-GPS) positioning. However, GPS and A-GPS receivers are not necessarily available in all wireless UEs, and some wireless communications systems do not support A-GPS. Furthermore, GPS-based positioning may often have unsatisfactory performance in urban canyons and indoor environments. There is, therefore, a need for complementary terrestrial positioning methods.
There are a number of existing different terrestrial positioning methods. One example is Observed Time Difference of Arrival (OTDOA) in LTE. Three key network elements in a LTE positioning architecture are the Location Services (LCS) Client, the LCS target, and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target by collecting measurements and other location information, assisting the UE in performing measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets. The LCS target is the entity that is being positioned. LCS Clients may reside in the LCS targets themselves. In the positioning procedure, a LCS Client sends a positioning request to a LCS Server to obtain location information, and the LCS Server processes and serves the received request and sends the positioning result and, optionally, a velocity estimate to the LCS Client. The positioning request may originate from the UE or the network.
In LTE, there exist two positioning protocols operating via the radio network: the LTE Positioning Protocol (LPP) and the LTE Positioning Protocol annex (LPPa). The LPP is a point-to-point protocol between the LCS server and the LCS target, used for the positioning of the LCS target. LPP may be used both in a user plane and in a control plane positioning procedure; and multiple LPP procedures are allowed in series and/or in parallel, thereby reducing latency. LPPa is a protocol between the eNodeB and the LCS server specified only for control plane positioning procedures, although it still may assist user plane positioning by the querying of eNodeBs for information and measurements. A Secure User Plane Location (SUPL) protocol is used as a transport protocol for LPP in the user plane.
Position calculation can be conducted, for example, by a positioning server (e.g., Evolved Serving Mobile Location Center (E-SMLC) or SLP in LTE) or the UE. The latter corresponds to the UE-based positioning mode, while the former may be network-based positioning (calculation in a network node based on measurements collected from network nodes such as Location Measurement Units (LMUs) or eNodeBs) or UE-assisted positioning (calculation is performed in a positioning network node based on measurements received from the UE).
FIG. 1 illustrates the Uplink Time Difference of Arrival (UTDOA) architecture being currently discussed in 3GPP. Although uplink (UL) measurements may in principle be performed by any radio network node (e.g., eNodeB), UL positioning architecture may include specific UL measurement units (e.g., LMUs) which, e.g., may be logical and/or physical nodes, may be integrated with radio base stations or sharing some of the software or hardware equipment with radio base stations, or may be completely standalone nodes with their own equipment (including antennas). The architecture is not yet finalized, but there may be communication protocols between the LMU and the positioning node, and there may be some enhancements for LPPa or similar protocols to support UL positioning. A new interface, SLm, between the E-SMLC and LMU, is being standardized for uplink positioning. The interface is terminated between a positioning server (E-SMLC) and the LMU. It is used to transport LMUp protocol (a new protocol being specified for UL positioning, for which no details are yet available) messages over the E-SMLC-to-LMU interface. Several LMU deployment options are possible. For example, a LMU may be a standalone physical node, it may be integrated into an eNodeB, or it may be sharing at least some equipment, such as antennas, with an eNodeB—these three options are illustrated in FIG. 1.
LPPa is a protocol between an eNodeB and a LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. In LTE, UTDOA measurements, UL RTOA, are performed on Sounding Reference Signals (SRS). To detect a SRS signal, a LMU needs a number of SRS parameters to generate the SRS sequence which is to be correlated to receive the signals. SRS parameters would have to be provided in the assistance data transmitted by the positioning node to the LMU, and this assistance data would be provided via LMUp. However, these parameters are generally not known to the positioning node, which needs them for obtaining information from the eNodeB that is configuring the SRS to be transmitted by the UE and to be measured by the LMU. This information would have to be provided in LPPa or a similar protocol.
The contents and composition of the assistance data/parameters to be provided to the LMUs by a positioning node is currently being discussed in 3GPP. A list of parameters, such as the examples shown in Table 1 below, may be signaled from the eNodeB to the positioning node and from the positioning node to the LMU. Further information on these example parameters can be found in the 3GPP submission: R2-121030, CR 36.305, Network Based Positioning Support, RAN WG2, February 2012.
TABLE 1First example list of parameters to support UTDOA:Parameter CategoryParametersGeneralC-RNTIServing eNB eCGI, PCIUL-EARFCNCyclic prefix ConfigUL-BandwidthSRSBandwidthSub-frame configurationFrequency domain positionCyclic shiftDurationTransmission combConfiguration indexMaxUpPts
Table 2 illustrates another, more detailed, example list of parameters that may alternatively be used. In general, any suitable list of parameters appropriate for the relevant network may be used.
TABLE 2Second example list of parameters to support UTDOA:ParameterCategoryParametersGeneralC-RNTIPCI of PCellNote1UL-EARFCN of PCellSRSFor each serving cell in which SRS is configured Note2:PCIUL-EARFCNDuplex mode configurationCyclic prefix ConfigReference ID or code for UE-specific SRS Note3Number of ports for SRS transmissionUL system bandwidth of the cellCell-specific SRS bandwidth configurationUE-specific SRS bandwidth configurationSRS subframe configurationFrequency domain positionSRS frequency hopping bandwidth configurationCyclic shiftTransmission combSRS configuration indexMaxUpPtsNote 1Including PCell should not imply configuring SRS on PCellNote 2 Multiple serving cells are possible for a UE configured with CANote 3 If agreed by RAN1 to be used in Rel-11
A positioning result is a result of the processing of the obtained measurements, including Cell IDs, power levels, received signal strengths, etc., which may be exchanged among nodes in one of the pre-defined formats. The signaled positioning result is represented in a pre-defined format corresponding to one of the seven Universal Geographic Area Description (GAD) shapes. Positioning result may be signaled between, e.g.:                LCS target (e.g., UE device) and LCS server, e.g., over LPP protocol;        Positioning servers (e.g., E-SMLC and SLP), over standardized or proprietary interfaces;        Positioning server and other network nodes (e.g., E-SMLC and MME/MSC/GMLC/O&M/SON/MDT);        Positioning node and LCS Client (e.g., between E-SMLC and PSAP or between SLP and External LCS Client or between E-SMLC and UE).        
In emergency positioning, the LCS Client may reside in Public Safety Answering Points (PSAPs). Positioning results are often based on radio measurements (e.g., timing measurements, such as timing advance and RTT, or power-based measurements, such as received signal strength) received from measuring radio nodes (e.g., UE or eNodeB or LMU).
As the name suggests, measurements for UL positioning and UTDOA are performed on UL transmissions, which may comprise, e.g., reference signal transmissions or data channel transmissions. Uplink Relative Time of Arrival (UL RTOA) is the currently standardized UTDOA timing measurement. This measurement may be performed on Sounding Reference Signals (SRS), which may be configured for periodic transmission. SRS transmissions may be triggered by any of the two trigger types:                Trigger type 0: higher layer signaling;        Trigger type 1: DCI formats 0/4/1A for FDD and TDD and DCI formats 2B/2C for TDD.        
UL positioning measurement performance may significantly degrade if the measuring node, in at least some pre-scheduled measuring occasions, tries to perform measurements on a signal which has not been transmitted (e.g., the signal may be pre-scheduled but not transmitted for some reason, and the measuring node may not be aware of this). Ideally, for the best measurement performance (e.g., accuracy), a measuring node should be aware of all pre-scheduled measurement occasions where the signal to be measured is actually present. However, this would imply providing to the measuring node detailed scheduling information, which may be rather dynamic and may cause high signaling overhead, particularly when the scheduling information is generally not available at the measured node (e.g., when the measuring node is not the scheduling node; for example, the LMU may be not integrated into the eNodeB and may be a separate node).
As an alternative, the transmissions to be measured may be configured persistently or semi-persistently, i.e., with a less dynamic configuration, which may be, e.g., periodic scheduling. This minimizes the signaling overhead of communicating the scheduling-related information, but typically leads to less efficient resource utilization. Another alternative for reducing signaling overhead is to perform blind signal detection while performing a measurement, which, however, is more complex and resource-demanding for the measuring node and typically degrades measurement quality (e.g., may take a longer time to measure or degrades measurement accuracy).